Internal combustion engines employing pistons have one or more cylinders, inside which a piston makes an alternating movement. The piston compresses the air or air-fuel mixture (depending on the engine in question) and the piston head or top (its upper part) receives the direct impact of the explosion of the air-fuel mixture. In the case of engines that operate according to the Diesel cycle, the stress to which the piston top is subjected is even greater, given the high compression rations required for ignition of the air-fuel mixture.
At present, the more and more restrictive limits of consumption, emission of pollutants and noises to which the makers of vehicles such as automobiles, utility vehicles and heavy-duty commercial vehicles are subjected are leading to the need to design smaller and smaller and more economic engines, which are, however, capable of generating high torque and power values.
This is achieved by reducing the cubic capacity (piston displacement) of the engine (a concept called downsizing), coupled with the use of higher and higher overfeeding values (increase in turbo pressure) and the redesigning of the internal engine components. These new premises of design and operation generate a considerable increase in heat load on the piston heads, requiring the development of solutions that should guarantee high durability to the engine during its normal operation.
Besides the great stress to which a piston is subjected, due to the high compression ratios, an intrinsic characteristic of diesel engines is the existence of a specially designed recess at the piston top, which defines much of the volume of the combustion chamber.
These high thermal loads are especially critical in the border and bottom regions of the chamber, the recess being designed for enabling a greater combustion volume. And when they are made from light alloys such as aluminum alloys, the operation of the component may break by thermal fatigue these critical regions, may alter the explosion parameters and, in extreme cases, even interfere with the operation of the engine.
There are various ways to eliminate this drawback, by reinforcing the resistance of the piston head in the region of the combustion chamber, but none of them is as effective as the solution that was proposed by the applicant after long tests and studies.
Some solutions and are known at present and can be used for raising the resistance of the recess surface corresponding to the combustion chamber at the piston head.
Patent Document JP2007064129 relates to a process for making a piston groove more resistant by friction of a tool by relative rotational movement between the piston and the tool while the tool turns with respect to its own axis. The groove is formed in the friction area.
This document refers chiefly to the increase in resistance to wear caused by the rings on the piston groove. This improvement is achieved by friction of a non-consumable pin due to the refinement caused by intense friction, heat and plastic deformation of the material.
This prior art is important because it discloses that such a processing is already possible for piston aluminum allows, but it there is still no addition of any mechanical-reinforcement element, as required at the piston head.
The British Patent Document GB2367027 relates to an engine piston onto which a second material is applied over the piston base material. At least a part of the recess present at the piston head is formed, on part of which the second material is present. The region of the second material is formed on the piston base, where the friction heat is generated between both, plasticizing the material adjacent the tool. The continuous movement of the tool forms the second material and mixes it with the first material at the joining ends, keeping them joined together. In essence, this document refers simply to the welding of two different materials, even if they are of similar compositions, in order to form a piston head having greater strength. Even if the concept of composite is an alternative, the difference lies in the existence of a joint welded between different materials, not a modification of the substrate, as proposed by the applicant.
Patent Document W02008/131273 discloses a cryogenic method for increasing the interface between metal and metal oxide particles, which can increase the efficiency of the exothermal reaction between these materials. One of the exothermal reactions described is 2Al(s)+3CuO(s)→Al2O3(s)+3Cu(s), the other being 2Al(s)+3Cu2O(s)→Al2O3(s)+6Cu(s). Whatever the reaction that takes place, it is followed by the complementary reaction 2Al(s)+Cu(s)→Al2Cu(s).
European Patent Document EP0460901 relates to a method for obtaining a surface of metallic composite material on a convex surface like an engine piston. The method comprises rotating a non-consumable tool over a substrate containing a layer of hard ceramic particles, the friction and heat causing these particles to be incorporated into the substrate surface. This document, however, refers only to the incorporation of particles ex situ (previously sintered and deposited onto the surface), and does not make any mention of the presence of Al2Cu as a reinforcement element.
As one can see, none of the prior-art documents cited above discloses the solution developed by the applicant, which is an engine piston made of aluminum, improved by the fact that a pure-copper or oxidized coating is applied onto at least a part of its head or top, after which the non-consumable pin is rotated and concomitantly drawn over the copper-oxide coating, bringing about an exothermal reaction with the (aluminum-based) piston substrate, causing a homogeneous dispersion of fine and hard Al2Cu particles (theta phase) and Al2O3. In addition to the deposition, a great refinement of grains takes place, due to the resulting deformation and temperature.
Optionally, the processed substrate may be subjected to the forced cooling with liquid nitrogen, or a mixture of liquid nitrogen and methanol, or still other similar solutions, in order to obtain greater localized refinement of the substrate grains. Further, subsequent treatments of solubilization and precipitation are possible.
With regard to the prior art EP 0460901, the present invention has considerable differences, above all regarding the fact that the in situ particles existing therein have greater chemical compatibility with the substrate, are naturally smaller, without the need for intensive comminution (fragmentation). Since the synthesis takes place during the processing, there is no thermal dilation gradient between the particle and the matrix and, therefore, it provides greater loading of the particles and, as a result, lower strength performance.